Method for producing a test signal

ABSTRACT

An apparatus and method are described for producing a test signal with a desired signal-to-noise ratio on a selectable output frequency for measuring a transmission system. A carrier signal, generated from a carrier signal, and a noise signal are remixed with the carrier signal in a bandwidth equal to or smaller than a smallest signal bandwidth of the transmission system. The levels of the carrier signal and the noise signal are adjusted to a predetermined ratio based on a measurement of the remixed carrier signal and the remixed noise signal. The noise signal is attenuated and added to the carrier signal until the desired signal-to-noise ratio is attained.

FIELD OF THE INVENTION

The present invention relates to signal processing and more particularlyto a method and an arrangement for producing a test signal with apredetermined signal-to-noise ratio and any selectable output frequencyfor measuring transmission systems that can be operated with differentsignal bandwidths.

BACKGROUND OF THE INVENTION

With digital television signals, a variety of modulation formats areused today in the domain of cable, terrestrial and satellitetransmission systems. Due to the multiplicity of symbol rates of thedigital television signals, these digital television signals havedifferent signal bandwidths. For example, there are transmission systemsthat can be operated with signal bandwidths of 4 MHz, 6 MHz, 8 MHz or 27MHz.

For measuring such transmission systems test signals are required, whichare given a defined noise level, or, in other words, have a definedsignal-to-noise ratio C/N (C=carrier, N=noise) in decibels, and can beplaced anywhere in a predetermined frequency range. Such test signalshave to date been composed of separately generated modulated carriersignals and noise signals. However, the signal-to-noise ratio calculatedfrom them is encumbered with the errors of the two signals and someasurements with such conventional test signals are imprecise.

There is a need for a simpler method, and one better suited to practice,for generating such test signals and a simple arrangement for carryingout such a method.

SUMMARY OF THE INVENTION

This and other needs are addressed by the present invention, in which aprecisely calibrated signal-to-noise (C/N) test signal can be generatedon a freely selectable output frequency to a desired signal-to-noiseratio of, for example, 20 dB, by attenuating the noise signal with asimple calibration line. This approach can be implemented withstraightforward circuit technology. It is particularly advantageous tocalibrate automatically carrier signal and noise signal levels to anequal ratio with a corresponding regulating arrangement, so thecalibration is also constantly repeated during operation and anytemperature influences or scattering of the carrier signal or of thenoise signal are automatically corrected.

It has proved particularly advantageous to generate the carrier signalby modulation of the carrier frequency of a local oscillator and toremix both the carrier signal and the noise signal into the basic bandwith this same carrier frequency before comparing levels, so,irrespective of the adjusted carrier frequency of the carrier signal,the comparison of the levels and regulation of the levels take place atthe constant output frequency of the inverse mixer.

Accordingly, one aspect of the present invention is directed to methodfor producing a test signal with a desired signal-to-noise ratio on aselectable output frequency for measuring a transmission system. In thismethod, a carrier signal, generated from a carrier signal, and a noisesignal are remixed with the carrier signal in a bandwidth equal to orsmaller than a smallest signal bandwidth of the transmission system. Thelevels of the carrier signal and the noise signal are adjusted to apredetermined ratio (preferably one) based on a measurement of theremixed carrier signal and the remixed noise signal. The noise signal isthen attenuated and adding to the carrier signal until the desiredsignal-to-noise ratio is attained. In one embodiment, the carrier signaland the noise signal are remixed with the carrier signal to anintermediate frequency in the basic band.

Another aspect of the present invention relates to an apparatus forproducing a test signal with a desired signal-to-noise ratio on aselectable output frequency for measuring a transmission system. Theapparatus includes a first adjustable calibration line for generating anoise signal based on a noise source and a second adjustable calibrationline for attenuating the noise signal and adjustable to a desiredsignal-to-noise ratio. A level detector is provided for receiving thenoise signal and a carrier signal via a toggle switch, in which thefirst calibration line is controlled based on an output of the leveldetector so that a level of the noise signal is in a predetermined ratiowith a level of the carrier signal.

In various embodiments, the apparatus may also include a modulator forgenerating the carrier signal based on a carrier frequency of a localoscillator, a mixer, supplied with the carrier frequency of the localoscillator, arranged between the toggle switch and the level detector,and/or a fixed frequency band filter coupled before the level detector,in which the band filter has a bandwidth equal to or smaller than asmallest signal bandwidth of the transmission system.

Still other aspects, features, and advantages of the present inventionare readily apparent from the following detailed description, simply byillustrating a number of particular embodiments and implementations,including the best mode contemplated for carrying out the presentinvention. The present invention is also capable of other and differentembodiments, and its several details can be modified in various obviousrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawing and description are to be regardedas illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a schematic diagram of an embodiment of the present invention.

FIG. 2 is a frequency-response diagram for an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for producing a test signal are described. In thefollowing description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It is apparent, however, to oneskilled in the art that the present invention may be practiced withoutthese specific details or with an equivalent arrangement. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

FIG. 1 shows a basic circuit arrangement for automatically generating atest signal with predetermined signal-to-noise ratio C/N in dB, for usein measuring TV transmission systems that can be operated with differentsignal bandwidths, for example.

A carrier signal C is generated from a modulator MO based on a carrierfrequency output from a local oscillator LO and a modulation signal(fmod). The local oscillator LO can be tuned to any number of carrierfrequencies, and the modulator MO can be implemented, for example, as anormal IQ modulator. A noise source (not shown) supplies a noise signalN. In the depicted exemplary embodiment, the noise signal N is appliedto an adjustable calibration line E1. A similar calibration line (notshown) can also be provided for the carrier signal C.

A regulated noise signal N′, produced from the adjustable calibrationline E1, and the modulated carrier signal C are then supplied to a leveldetector D via a toggle switch U. The toggle switch U is controlled by aprocessor P in such a way that the carrier signal C and the noise signalN are alternately supplied to the level detector D via a mixer MI and aband pass filter BP. In the mixer MI, the carrier frequency of the localoscillator LO is supplied, so that both the carrier signal C and thenoise signal N′ from any frequency position are each inversely mixed, orremixed, with a fixed intermediate frequency, preferably in the basicband. The band pass filter BP may be tuned to a fixed transmissionfrequency, and its bandwidth B tuned to the smallest operable signalbandwidth of the transmission system. As a result, as shown in FIG. 2,both the noise signal N and the carrier signal C are measured onlywithin this measuring bandwidth B. (During the remixing into the basicband, the carrier frequency LO in FIG. 2 is cancelled out and becomeszero).

The levels of carrier signal C and noise signal N′, measured in this waywithin the bandwidth B, are compared by the processor P, which adjuststhe calibration line E1 so that C and N′ are in a predetermined ratio toone another. Preferably, this ratio is one (1), so that both levels aretherefore equal in size.

The regulated noise signal N′ is supplied to a second calibration lineE2, which can be attenuated to produce a noise signal N″ for a desiredsignal-to-noise ratio C/N″. The noise signal N″, attenuated to thisdesired ratio, is added to the carrier signal C in an adder A. Theoutput signal of the adder A is then the test signal with thepredetermined signal-to-noise ratio C/N″.

The bandwidth B of the band pass filter BP can preferably be reversedand thus adjusted to the signal bandwidths. In the arrangement of thetoggle switch U in front of the adder A, illustrated in FIG. 1,calibration is done automatically during operation without interruptionof the generated test signal.

In many cases, however, it can be advantageous to interrupt generatingthe test signal during calibration, for example, if potential errors ofthe second calibration line E2 or the adder A are to be detected at thesame time by the calibration. In this embodiment, the output signal ofthe adder A is supplied directly to the input of the inverse mixer M1and via a selective switch in the carrier signal path C and the noisesignal path N alternately only the carrier signal C and only the noisesignal N″ are supplied to the adder and their levels are then comparedin the level detector D, as described above. The calibration line E2must in this case be adjusted to a dividing ratio of one (1).

Furthermore, it may be advantageous to provide these two operatingpossibilities simultaneously in one device, to enable calibration duringoperation or with interruption.

While the present invention has been described in connection with anumber of embodiments and implementations, the present invention is notso limited but covers various obvious modifications and equivalentarrangements, which fall within the purview of the appended claims.

What is claimed is:
 1. A method for producing a test signal with a desired signal-to-noise ratio on a selectable output frequency for measuring a transmission, said method comprising: remixing a carrier signal, generated from a carrier frequency, and a noise signal with the carrier frequency in a bandwidth equal to or smaller than a smallest signal bandwidth of the transmission system; regulating levels of the carrier signal and the noise signal to a predetermined ratio based on a measurement of the remixed carrier signal and the remixed noise signal; and attenuating the noise signal and adding the attenuated noise signal to the carrier signal until the desired signal-to-noise ratio is attained.
 2. A method according to claim 1, wherein the predetermined ratio is one (1).
 3. A method according to claim 1, wherein said attenuating is performed by means of an adjustable calibration line.
 4. A method according to claim 1, further comprising modulating a local carrier signal to generate the carrier signal, wherein the carrier signal and the noise signal are remixed with the local carrier signal to an intermediate frequency in a basic band.
 5. A method according to claim 1, wherein said regulating the levels is performed with or without interruption of generation of the test signal.
 6. A method according to claim 1, wherein said regulating the levels is performed by regulating the level of the noise signal and/or the level of the carrier signal.
 7. A method according to claim 1, wherein said regulating the ratio of the levels is performed automatically.
 8. An apparatus for producing a test signal with a desired signal-to-noise ration on a selectable output frequency for measuring a transmission system, said apparatus comprising: a first adjustable calibration line for generating a noise signal based on a noise source; a level detector receiving the noise signal and a carrier signal via a toggle switch, wherein the first calibration line is controlled based on an output of the level detector so that a level of the noise signal is in a predetermined ratio with a level of the carrier signal; and a second adjustable calibration line for attenuating the noise signal and adjustable to the desired signal-to-noise ratio.
 9. An apparatus according to claim 8, further comprising: the wanted signal (C) is generated by modulation of the carrier frequency of a local oscillator (LO) in a modulator (MO) and an inverse mixer (MI), supplied with the frequency of the local oscillator (LO), is arranged between the reversing switch (U) and the level detector (D).
 10. An apparatus according to claim 8, further comprising a fixed frequency band filter coupled before the level detector, said band filter having a bandwidth (B) equal to or smaller than a smallest signal bandwidth of the transmission system. 